1. Field of the Invention
The present invention relates to a portable terminal, and more particularly to a keypad lighting device.
2. Description of the Related Art
In general, portable terminals, such as portable computers and mobile communication terminals, include a physical keypad as an input device. Typically such a portable terminal is provided with a lighting device in the keypad to allow a user to easily operate the input device in a using environment, in particular even at night or inside a poorly illuminated room. Terminals that are usually used while being carried by a user, for example, mobile communication terminals, have already been equipped with a keypad lighting device, and also terminals that are typically not carried by a user, for example, portable computers, have begun to be provided with a keypad lighting device, recently. Such a keypad lighting device is also supplied in a wired telephone, a digital door lock, a remote controller for a home appliance or the like.
A common mobile communication terminal is equipped with a physical keypad including key tops in a 3×4 arrangement. Such a physical keypad generally adopts a direct type lighting method. That is, light emitting devices are generally arranged in such a manner that each of the light emitting devices corresponds to one key top, or one light emitting device corresponds to every two or four key tops. As the performances of light emitting devices, such as light emitting diodes, are improved and lighting technologies are developed, edge type lighting devices have become commercially available, in which the edge type lighting devices employ a waveguide film as a light guide plate member. An edge type lighting device includes a waveguide film arranged over the entire area of a keypad, and plural light emitting devices arranged along the edges of the waveguide film, so that the light projected from the light emitting devices can be diffused over the entire area of the keypad through the waveguide film.
As the edge type lighting devices become commercially available, it becomes possible to reduce the space required for installing a keypad and a keypad lighting device. As a result, it is easier to miniaturize a portable terminal by applying such an edge type key pad lighting device. Consequently, such a lighting device is also provided in a keyboard of a portable computer or the like. This is because a lighting device employing a light guide plate allows a wider area to be uniformly lighted with a smaller number of light emitting devices.
Light emitting diodes are typically employed as the light emitting devices for such a keypad lighting device. Although a light emitting diode is suitable as a light emitting device for a keypad lighting device, the intensity of the light is gradually diminished as distance from a light emitting surface thereof increases. That is, there is a limit in angular extent and distance that can be directly lighted by a single light emitting diode.
FIG. 1 illustrates a graph 10 showing a distribution of luminous intensities of a light emitting diode measured while changing an angle in relation to the central axis of a light source, i.e. an optical axis O. FIG. 2 illustrates a graph 20 showing a distribution of luminous intensities of a light emitting diode measured while changing a distance from the light source. FIG. 1 shows ratios of luminous intensities depending on the angle in relation to the optical axis O assuming that the luminous intensity measured on the optical axis O of the light emitting diode is “1,” and FIG. 2 shows ratios of luminous intensities depending on the distance from the light source, i.e., a light emitting surface of the light emitting diode assuming that the luminous intensity measured on the light emitting surface of the light emitting diode is “1.” From FIGS. 1 and 2, it can be appreciated that no illumination is provided in a range deviated by about 60 degrees from the optical axis O, and luminous intensities are gradually lowered as distance from the light emitting surface increases.
FIG. 3 illustrates a keypad 100 of a portable terminal that includes a conventional edge type lighting device. As shown, the keypad 100 includes a keypad member 101 provided with plural key tops 111, and a dome sheet 102 arranged below the keypad member 101.
The dome sheet 102 may be configured by a flexible printed circuit board 121, which is provided with plural dome switches 123 arranged on one side thereof to correspond to the key tops 111, respectively. A waveguide film 103 as a light guide plate member, and light emitting devices 131 are interposed between the dome sheet 102 and the keypad member 101. The light emitting devices 131 are arranged adjacent to an outermost edge of the waveguide film 103, and the light projected from the light emitting devices 131 are adapted to be capable of lighting the entirety of the keypad member 101 through the waveguide film 103.
The keypad lighting device employing the light emitting devices 131 and the waveguide film 103 as described above may be configured using a substantially reduced number of light emitting devices as compared to the number of the key tops 111.
FIG. 4 exemplifies a configuration of a conventional edge type keypad lighting device, in which a light emitting surface of the light emitting device 131 and a light-incident surface 133 of the waveguide film 103 are arranged parallel to each other. The light projected from the light emitting device 131 is incident to the waveguide film 103 through the light-incident surface 133 after passing through an air layer. The light suffers from refraction while passing through the light-incident surface 133 due to the difference of density between the air layer and the waveguide film 103. Due to the refraction caused by the difference of light propagation media, the angular range to be capable of being lighted by the light incident to the waveguide film will be reduced as compared to the angular lighting range of the light emitting device 131. As a result, the conventional edge type keypad lighting device has a limit in uniformly lighting the entire area of a keypad while reducing the number of light emitting devices.
FIG. 5 illustrates a dome sheet 223 of a keypad that is provided with a conventional direct type keypad lighting device. As shown, it can be seen that the dome sheet 223 is attached on the top of a rigid printed circuit board 221, and plural dome switches 225 are arranged on a surface thereof. It can also be seen that each of the light emitting devices 231 of the keypad lighting device is arranged between the dome switches 225 on the printed circuit board 221.
A keypad, to which a direct type keypad lighting device is applied, employs a silicon pad as a light guide plate member, and key tops are provided on the top surface of the silicon pad, in which the key tops are arranged at the positions corresponding to the dome switches 225, respectively.
FIG. 6 illustrates a light guide plate member 204 employed in a conventional direct type keypad lighting device. As shown, the light guide plate member 204 is formed with plural protrusions 241 on the bottom side thereof, and the protrusions 241 are formed with actuation bumps 243, respectively, in which the actuation bumps 243 correspond to the dome switches 225, respectively. When the light guide plate member 204 is arranged on the dome sheet 223, each of the actuation bumps 243 is arranged to be in alignment with one of the dome switches 225, and in accordance with a user's operation, the actuation bumps 243 directly actuate the dome switches 225, respectively. On the top of the light guide plate member 204, key tops are attached, which are arranged to the positions corresponding to the protrusions 241, respectively. When the light guide plate member 204 is arranged on the printed circuit board 221, each of the light emitting devices 231 is arranged between the protrusions 241 to light one protrusion or two or more protrusions adjacent thereto, and consequently to light the corresponding key top(s).
In the conventional direct type keypad lighting device as described above, the light of the light emitting devices is incident to the protrusions through the light guide plate member. However, since the positions of the protrusions in relation to the light emitting devices are different from each other, the light-incident angles and distances are different from protrusion to protrusion. In addition, some protrusions may be lighted by one light emitting device, and some protrusions may be lighted by two or more light emitting devices. As a result, the conventional direct type keypad lighting device suffers from substantial difficulty in uniformly lighting all the key tops. Furthermore, since the surfaces of a single protrusion, to which the light of one or more lighting or light emitting devices is incident, are various, locally bright and dark areas are produced even in one key top.